In general, in an electrophotographic recording method as a recording method for a PPC, scanning is performed with a laser to form a latent image on an intermediate recording medium. The latent image is subjected to toner development to achieve image formation. In electrophotographic recording, the boundary between an image portion and a non-image portion is an intermediate transit area where toner development is unstable and exposure with a laser and toner development of a latent image are insufficient.
With respect to pixels which are consecutive in the main scanning direction, a method is taken in which pixel modulation which exposes pixels subjected to pulse width modulation is carried out for every several pixels together by laser driving, thereby eliminating an image portion and a non-image portion so that image formation is stabilized.
For example, in conventional three-pixel modulation, pixel values of two pixels among three consecutive pixels in the main scanning direction are added to concentrate the pixel values on one pixel.
However, in cases where pixel modulation (particularly three or more pixels) according to the conventional method is carried out on an error-diffused image of a multi-value level which is as low as about four to eight values, the pulse width is so coarse that continuity of pixels in the sub-scanning direction is difficult to maintain. Therefore, the screen-line structure has no smoothness so that the structure comes to have conspicuous boundaries between image portions and non-image portions.
In addition, the pixel density is low in a high-light portion so that the structure comes to be particularly unstable.
In this respect, as a relevant technique, Jpn. Pat. Appln. KOKAI Publication No. 10-304189 discloses a technique concerning an image forming apparatus and an image forming method, which subjects multi-value data to pulse width modulation in units of multiple pixels using a dither matrix having a threshold level which changes in synchronization with the screen-line position, thereby forming a screen-line structure.